The present invention relates to an analog circuit constituted by an operational amplifier, resistors, and capacitors and, more particularly, to an analog circuit having a full differential arrangement arranged on a semiconductor substrate.
A differential type analog circuit of this type is used in an active filter or the like. For example, in the analog interface unit of A/D and D/A converters for converting an analog signal to a digital signal and converting a digital signal to an analog signal, a low-pass active filter of this type is used as a pre-filter used for reducing a folded signal normally generated at about a sampling frequency on the A/D converter side, or as a post-filter used for reducing out-band quantization noise or video noise generated on the D/A converter side.
FIG. 4 shows a conventional low-pass active filter. Referring to FIG. 4, reference number 1a denotes an operational amplifier; C.sub.14 and C.sub.2, capacitors; and R.sub.1 and R.sub.2, resistors.
Assuming that the operational amplifier 1a has ideal characteristics, an output voltage generated at an output terminal OUT with respect to an input voltage Vi applied to an input terminal IN is represented by Vo, and the capacitance of the capacitor C.sub.14 is represented by 4C.sub.1. At this time, a transfer function of the input and output voltages Vi and Vo can be given by equation (1). EQU Vo/Vi=-1/{4R.sub.1 R.sub.2 C.sub.1 C.sub.2 s.sup.2 +(2R.sub.1 C.sub.2 +R.sub.2 C.sub.2)s+1} (1)
When the low-pass active filter shown in FIG. 4 is realized on a semiconductor substrate to achieve the above-mentioned object, the following problems are posed. That is, a power-supply voltage variation elimination ratio is low, and when a semiconductor substrate voltage varies, an output from the low-pass active filter varies due to capacitive coupling.
As a conventional method of solving the above problems to reduce noise, the following method has been used. That is, the above single-output type low-pass filter is changed into a full differential type filter by using a differential output type operational amplifier.
FIG. 5 shows a filter circuit obtained by changing the low-pass active filter shown in FIG. 4 into a full differential filter.
This circuit is constituted by: a differential output type operational amplifier 1; a first network A connected between the inverting input terminal and non-inverting output terminal of the operational amplifier 1 and constituted by a capacitor C.sub.2 and resistors R.sub.1 and R.sub.2 ; a second network B connected between the non-inverting input terminal and inverting output terminal of the operational amplifier 1 and constituted by a capacitor C.sub.2 and resistors R.sub.1 and R.sub.2 ; and a third network C constituted by a single capacitor C.sub.12 for connecting the first network A to the second network B.
In the circuit in FIG. 5, when the capacitance of the capacitor C.sub.12 is set to be 2C.sub.1, the transfer function of the circuit at input terminals IN.sub.1 and IN.sub.2 and output terminals OUT.sub.1 and OUT.sub.2 is given by equation (1).
When the full differential type analog circuit arranged as described above is employed, a transfer function from a power supply line or the semiconductor substrate to the non-inverting output terminal is equal to that from the power supply line to the inverting output terminal, the outputs from the non-inverter and inverting output terminals cancel out, and the variation in output detected in a single-output arrangement does not occur.
Therefore, noise can be reduced by 40 to 60 dB when the full differential type analog circuit is used compared with the single-output arrangement.
Conventionally, in a full differential type analog circuit manufactured on a semiconductor substrate, a capacitor, as shown in FIG. 6A, is arranged on a semiconductor substrate 2 such that an insulating film 5 is inserted between a lower capacitor electrode 3 and an upper capacitor electrode 4.
In this case, parasitic capacitances viewed from both the upper capacitor electrode 4 side and the lower capacitor electrode 3 side will be considered. Assuming that the capacitance of this capacitor is set to be C.sub.1, a parasitic capacitance 6 shown in FIG. 6B and generated between the lower capacitor electrode 3 and the semiconductor substrate 2 is about C.sub.1 /10, and a parasitic capacitance generated between the upper capacitor electrode 4 and the semiconductor substrate 2 can be neglected compared with the parasitic capacitance between the lower capacitor electrode 3 and the semiconductor substrate 2.
This difference between the parasitic capacitances, in the full differential type low-pass filter shown in FIG. 5, causes a difference between influences of the semiconductor substrate 2 on the transfer functions of a non-inverting output and an inverting output, thereby occurring a variation in differential voltage.